docs: update changelog since v1.0.0

[berisher]

Summary

• preserve the existing changelog sections as the 1.0.0 release notes instead of rewriting them
• add a separate 1.0.1 entry that captures the meaningful changes merged after the v1.0.0 tag
• keep the release links aligned with the actual v1.0.0 tag that was published on 2025-12-23

Changes

• move the prior top-level Added/Changed/Fixed release notes under ## [1.0.0] - 2025-12-23 without rewriting those bullets
• add a new ## [1.0.1] - Unreleased section covering the post-v1.0.0 workflow hardening, generator contract, validation, mutator, example, and packaging changes
• keep ## [Unreleased] empty for future work after 1.0.1
• update the changelog footer links so both Unreleased and 1.0.1 compare from v1.0.0 until the 1.0.1 tag exists

How to validate

• git diff --check